FIELD OF THE INVENTION
The present invention relates to a circuit for producing a reference voltage or to a reference-voltage source, including a first circuit device for producing a first voltage having a negative temperature coefficient, and a second circuit device for producing a difference voltage from a second voltage and a third voltage, the second voltage and the third voltage are each derived from forward voltages across corresponding pn junctions, the difference voltage is subject to a positive temperature coefficient, and the reference voltage may be tapped off as a sum of the first voltage from the first circuit device and the difference voltage from the second circuit device.
Most integrated circuits operated from an unstabilized supply voltage, that is to say virtually all smart power ICs, require an internal reference voltage source. That is particularly true of voltage regulators having an output voltage which is used by other integrated circuits or circuit blocks as a reference voltage.
Known reference voltage sources use zener diodes, for example, which are supplied with an unstabilized input voltage through a series resistor. A voltage tapped off the zener diode is used as a stabilized reference voltage. In addition, it is possible, in principle, for the forward voltage across a diode or the base/emitter voltage of a bipolar transistor to be used generally as a reference voltage. However, the forward voltage across a pn junction has a negative temperature coefficient and therefore a temperature dependency which has a negative effect for a large number of applications. If, for example, a voltage regulator having an output voltage which is used as a reference voltage is intended to be used to supply sensors, A/D converters or similar components, the output voltage of the voltage regulator must be very precise and, in a particular, extremely temperature-stable. In that context, tolerance limits of up to a maximum of 1% are normal requirements today.
For that reason, the reference voltage sources described above have in recent years been superseded by bandgap reference voltage sources, which provide a temperature-stabilized reference voltage. Those known bandgap reference voltage sources are based on addition of a forward voltage across a pn junction through which current flows and a difference voltage which is multiplied by a corresponding factor and is formed from two forward voltages across two pn junctions that have different current densities flowing through them. In general, the forward voltage across a pn junction with current flowing through it, as already explained above, has a negative temperature coefficient. In contrast, the difference between the two forward voltages rises in proportion to the absolute temperature and is therefore subject to a positive temperature coefficient. If the factor by which the difference voltage explained above is multiplied is set in such a way that the negative temperature coefficient of the forward voltage across the pn junction cancels out the positive temperature coefficient of the difference voltage, it is possible to achieve a temperature-stabilized output or reference voltage which is then a parabolic or square function of temperature. In particular, the output voltage of the bandgap reference voltage source, which is obtained by adding the forward voltage (explained above) across a pn junction through which current flows to the difference voltage, multiplied by the corresponding factor, formed by two further forward voltages, is approximately 1.25 V, which is roughly equivalent to the bandgap of silicon. The magnitude of the output voltage of that reference voltage source has therefore lent its name to the bandgap reference voltage source.
A generalized circuit diagram of a known bandgap reference voltage source is shown in FIG. 2 and described in detail below. In that device, resistor ratios, a current-mirror transmission ratio and a ratio of emitter areas of transistors are particularly critical for achieving a tight tolerance for an output voltage. That circuit also reacts very sensitively to temperature gradients widely encountered in integrated power circuits. Accordingly, it is necessary to configure the transistors in an implemented circuit layout exactly on isotherms from the greatest heat source in the appropriate circuit. However, a modern layout with reusable circuit and layout blocks prevents the circuit from being adapted to suit the particular position of the available heat sources. Furthermore, the number of heat sources in smart power ICs is constantly increasing, so that the course of the corresponding isotherms from those heat sources cannot be determined clearly. The multiplicity of components having pairing properties which are critical in the bandgap reference voltage source also generally necessitates individual adjustment of the circuit. That can be carried out, for example, by using so-called "zapping" zener diodes, that break down and produce a low-resistance connection when a high external voltage is applied in the reverse direction. However, that increases the technical complexity.